13 cis-retinoid acid (13-CRA), a compound previously demonstrated to have antitumor activity in limited clinical trials, is known to have a variety of effects on the immune system. The compound was investigated in a Phase I clinical trial at three dose levels to determine whether there were any biological response modifying effects when administered to cancer patients. No unexpected clinical toxicities were observed and there were no changes in immune function observed at any dose level despite intensive monitoring of natural killer cell activity, monocyte function, lymphoproliferative responses, and leukocyte populations. There were no tumor responses observed during the time of treatment with 13-CRA.